1. Field of the Invention
This invention relates generally to a phase locked loop circuit, and more particularly is directed to a phase locked loop circuit which has no unlocking state.
2. Description of the Prior Art
In the prior art, phase locked loop circuits have been provided which comprise a variable frequency oscillator, a reference signal oscillator, a phase comparator and a low pass filter. If such a phase locked loop circuit is employed in a local oscillation circuit of an FM radio receiver, the receiver's receiving property or the stability thereof is improved with less frequency drift due to temperature variation or time lapse.
Such phase locked loop circuits are of two types, in one of which a 1/N frequency divider is employed for varying the frequency of the variable frequency oscillator, and in the other of which a manually variable capacitor is used for the same object. Recently, since the phase locked loop circuits using a 1/N frequency divider are complicated in circuit construction and, hence, relatively costly, the phase locked loop circuits using a manually variable capacitor are becoming increasingly popular.
In phase locked loop circuits of the type in which the frequency f.sub.S of the variable frequency oscillator is varied by a variable capacitor, if the frequency of the reference signal oscillator is f.sub.R and the order of high harmonics is N, the frequency f.sub.S is expressed as f.sub.S =N.multidot.f.sub.R. Thus, a desired frequency for local oscillation is obtained by selecting the order N by means of the variable capacitor.
With the above described prior art phase locked loop circuit, when the relation (f.sub.C +f.sub.L)&lt;f.sub.R is established between the lock range f.sub.L of the phase locked loop, its capture range f.sub.C and the frequency f.sub.R of the reference signal oscillator, an unlocking state may be encountered at a predetermined position of the variable capacitor. When such an unlocking state occurs, an unnecessary and undesirable noise is produced in the output of the receiver.
Although a receiver which employs the prior art phase locked loop circuit in its local oscillation circuit has been provided with a muting circuit for eliminating a noise between broadcasting stations, such existing receiver does not provide a circuit for positively detecting a muting control signal for the muting circuit.